1. Field of the Invention
This invention relates to material removal using focused particle beams and, more particularly, to mask modification by using focused ion beams or electron beams.
2. Description of the Related Art
Integrated circuit fabrication generally employs lithographic processes to form circuit elements on the surface of a semiconductor wafer. In many lithography processes, a mask shapes an exposing energy source to form patterns in a resist which is sensitive to that energy source. The exposing energy source can be, for example, ultraviolet light, x-rays, ion beams, or electron beams selected according to the device being formed and the desired resolution.
The design of the mask depends upon the exposing energy source for lithography. In optical lithography, masks can be glass or quartz covered with a material which absorbs light such as chromium, chromium oxide, iron oxide, or silicon. In x-ray lithography, an x-ray absorbing material such as gold or tungsten is patterned on an x-ray transmitting membrane such as silicon, silicon nitride, silicon carbide, aluminum oxide, polyimide, and combinations of these layers. In SCALPEL (SCattering with Angular Limitation Projection Electron Lithography), masks are used to interact with electrons for resist exposure. Detailed information concerning lithography and mask formation is found in S. M. Sze, Ed., VLSI Technology, (McGraw-Hill Book Company, New York), c. 1988 and L. F. Thompson, Ed., Introduction to Microlithography, (American Chemical Society, Washington, D.C.), c. 1983, the disclosures of which are incorporated by reference. Further information on SCALPEL lithography is found in Berger et al., U.S. Pat. No. 5,079,112, the disclosure of which is incorporated by reference.
For reliable lithography processing, the mask must be essentially free from defects. Defects in the mask pattern generally fall within one of two categories: clear defects and opaque defects. Clear defects occur when a portion of the mask pattern is missing while opaque defects occur when excess material is present.
Defect repair entails deposition of additional material to fill in clear defects and removal of material to eliminate opaque defects. In the past, material has been removed by focused ion beams to directly sputter away the opaque detect. There are several problems with this approach. First, the sputter rate of a polycrystalline material is dependent upon its orientation with respect to the ion beam. Grains with low index planes oriented parallel to the ion beam have significant ion channeling resulting in a low sputter rate. Grains whose planes are misoriented, i.e., whose planes are not perfectly parallel to the ion beam but are impinged by the beam at an angle, have a higher sputter rate. For a polycrystalline sputter target, grains which are misoriented with respect to the ion beam are preferentially sputtered. As a result of this preferential sputtering, a rough residue of unsputtered material remains on the layer or substrate underneath after high sputter rate grains have been removed. This phenomenon is shown in FIG. 1, which depicts tungsten residue following sputtering with a Ga.sup.+ focused ion beam. Further sputtering to remove this residue leads to undesirable substrate etching.
The rough residue resulting from preferential sputtering also creates problems at the sidewalls of the region being etched. For thicker layers of material, grain sizes are usually larger due to grain growth during layer buildup. When some grains are preferentially sputtered, the edges of remaining grains create a rough line. For example, in tungsten absorber x-ray masks, tungsten grain sizes for a 3000 .ANG. film are approximately 0.15 micron. Because desired feature resolutions are on the order of 0.25 micron, sidewall roughness of 0.15 micron for tungsten x-ray absorber results in unacceptable image production. Generally, roughness should be no more than approximately 10% of the feature size.
Another problem associated with ion beam sputtering is redeposition of the sputtered material on nearby features. Redeposition is especially a problem for opaque defect removal on x-ray masks due to the large aspect ratio (absorber thickness to feature size) and dense patterns found on x-ray masks. The redeposited material blurs pattern features, e.g., by depositing on sidewalls, resulting in poor image quality.
Thus there is a need in the art for improved etching processes which can locally remove portions of a material layer without the anisotropy and resputtering associated with physical ion beam etching. Such a technique could be used for fine etching and mask modifications, particularly the removal of opaque mask defects.